Lets talk accessory work!
Accessory exercises serve many purposes.
For one, they allow you to continue to train after you finish your primary exercise or exercises. Just because you're slightly fatigued doesn't mean you shouldn't continue training, and accessory exercises offer you the chance to do that without the same risk of injury/overuse you'd be exposed to if you just performed primary exercises over and over at a high intensity.
Accessory exercises also allow you to focus on different muscle groups and or hit certain muscle groups in different ways other than the primary exercises. This ultimately reduces your weakness and imbalances and makes you a stronger and more resilient in the gym. When used properly, not only will accessory exercises make you stronger and more adaptable to a variety of movement patterns, but they'll also help you get better and stronger at your primary exercises